This invention relates to a system for providing a flow of hydraulic fluid to the steering assembly of a self-propelled, ground contacting vehicle. More particularly, this invention relates to a system for providing a supplemental or emergency flow of hydraulic fluid to the hydraulic steering assembly of a large, self-propelled vehicle, such as a construction machine.
Many types of off-road, self-propelled construction machines, such as graders and earth moving machines, and certain types of heavy, roadable machines, such as articulated loaders, are provided with hydraulically-operated steering systems. Typically, these hydraulically-operated steering systems utilize the vehicle engine as the source of power for the pump that is used to cause hydraulic fluid to flow through the steering system. However, the engine of such a vehicle is subject to accidental or inadvertent shutdowns at times when the vehicle is in motion, and the engine operated pump is subject to failure even when the engine continues to operate, for example, due to a coupling failure or a sheared drive shaft, and for safety reasons it is important to continue to hydraulically power the vehicle steering system whenever the vehicle is in motion, until it can be brought safely to a stop. This is frequently accomplished through the use of a wheel-driven or ground-driven supplemental or emergency hydraulic system, the use of such a system being characterized as an "emergency" system if it is only used during an engine off or pump failure situation, and as a "supplemental" system if it is also used to power the vehicle steering system during an engine on situation when the capacity of the primary or engine-driven hydraulic system is inadequate.
The use of a ground-driven emergency or supplemental hydraulic steering system for an off-road vehicle has heretofore significantly involved considerable complexity and expense, since such a system must be reversible to accommodate the fact that the vehicle may be moving in either a forward or a reverse direction when power from such a system is needed, and it must also be variable in capacity to deal with the fact that the ground speed of the vehicle is infinitely variable between maximum forward and reverse speeds at such times. These requirements have been met by various types of ground-driven emergency or supplemental hydraulic pumping systems, but the known types of hydraulic systems are costly and complex, involving the use of multiple check valves in order to be able to accommodate the reversible character of the prime mover for the pump for any such system, that is, the forward or reverse direction of motion of the vehicle. Thus, an emergency or supplemental system has heretofore been provided by the use of a ground-driven gear pump, in combination with four one-way or check valves, or by the use of a ground-driven, check ball, multi-piston piston pump, also in combination with two check valves for each piston of such pump. However, the use of such a large number of valves in a system of the foregoing type adds to the cost and the complexity of the system and introduces additional flow restrictions which limits the hydraulic capacity of the system.